Move sqlalchemy to snowflake optional dep

[korikuzma]

I'm not sure if we needed to add sqlalchemy to required dependencies (since not everyone will be using sqlalchemy). I think the optional snowflake dep handles this?

Originally posted by @korikuzma in #101 (comment)

Will need to update docs to show optional deps and how to install

[jsstevenson]

(It would also need to be in the Postgres group)

[korikuzma]

@jsstevenson Oh that's my bad for not being familiar with the source code. I didn't realize we were using sqlalchemy with the postgres backend. I just assumed we were only using psycopg2. I'll close